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LibWeb: Parse CSS background-position
property
This is done a bit differently from other properties: using a TokenStream instead of just a Vector of ComponentValues. The reason for this is, we can then use call the same function when parsing the `background` shorthand. Otherwise, we would have to know in advance how many values to pass down, which basically would involve duplicating the `background-position` parsing code inside `background`. The StyleValue is PositionStyleValue, since it represents a `<position>`: https://www.w3.org/TR/css-values-4/#typedef-position Unfortunately, background-position's parsing is a bit different from `<position>`'s, (background-position allows 3-value syntax and `<position>` doesn't) so we'll need to come back and write a different parsing function for that later.
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6 changed files with 260 additions and 2 deletions
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@ -2487,6 +2487,175 @@ RefPtr<StyleValue> Parser::parse_background_image_value(ParsingContext const& co
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return nullptr;
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}
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RefPtr<StyleValue> Parser::parse_single_background_position_value(ParsingContext const& context, TokenStream<StyleComponentValueRule>& tokens)
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{
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// NOTE: This *looks* like it parses a <position>, but it doesn't. From the spec:
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// "Note: The background-position property also accepts a three-value syntax.
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// This has been disallowed generically because it creates parsing ambiguities
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// when combined with other length or percentage components in a property value."
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// - https://www.w3.org/TR/css-values-4/#typedef-position
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// So, we'll need a separate function to parse <position> later.
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auto start_position = tokens.position();
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auto error = [&]() {
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tokens.rewind_to_position(start_position);
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return nullptr;
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};
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auto to_edge = [](ValueID identifier) -> Optional<PositionEdge> {
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switch (identifier) {
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case ValueID::Top:
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return PositionEdge::Top;
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case ValueID::Bottom:
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return PositionEdge::Bottom;
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case ValueID::Left:
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return PositionEdge::Left;
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case ValueID::Right:
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return PositionEdge::Right;
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default:
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return {};
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}
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};
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auto is_horizontal = [](ValueID identifier) -> bool {
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switch (identifier) {
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case ValueID::Left:
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case ValueID::Right:
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return true;
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default:
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return false;
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}
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};
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auto is_vertical = [](ValueID identifier) -> bool {
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switch (identifier) {
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case ValueID::Top:
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case ValueID::Bottom:
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return true;
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default:
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return false;
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}
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};
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auto zero_offset = Length::make_px(0);
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auto center_offset = Length { 50, Length::Type::Percentage };
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struct EdgeOffset {
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PositionEdge edge;
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Length offset;
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bool edge_provided;
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bool offset_provided;
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};
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Optional<EdgeOffset> horizontal;
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Optional<EdgeOffset> vertical;
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bool found_center = false;
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while (tokens.has_next_token()) {
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// Check if we're done
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auto seen_items = (horizontal.has_value() ? 1 : 0) + (vertical.has_value() ? 1 : 0) + (found_center ? 1 : 0);
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if (seen_items == 2)
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break;
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auto& token = tokens.peek_token();
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auto maybe_value = parse_css_value(context, token);
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if (!maybe_value || !property_accepts_value(PropertyID::BackgroundPosition, *maybe_value))
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break;
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tokens.next_token();
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auto value = maybe_value.release_nonnull();
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if (value->has_length()) {
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if (!horizontal.has_value()) {
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horizontal = EdgeOffset { PositionEdge::Left, value->to_length(), false, true };
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} else if (!vertical.has_value()) {
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vertical = EdgeOffset { PositionEdge::Top, value->to_length(), false, true };
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} else {
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return error();
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}
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continue;
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}
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if (value->has_identifier()) {
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auto identifier = value->to_identifier();
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if (is_horizontal(identifier)) {
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Length offset = zero_offset;
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bool offset_provided = false;
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if (tokens.has_next_token()) {
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auto maybe_offset = parse_length(context, tokens.peek_token());
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if (maybe_offset.has_value()) {
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offset = maybe_offset.value();
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offset_provided = true;
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tokens.next_token();
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}
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}
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horizontal = EdgeOffset { *to_edge(identifier), offset, true, offset_provided };
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} else if (is_vertical(identifier)) {
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Length offset = zero_offset;
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bool offset_provided = false;
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if (tokens.has_next_token()) {
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auto maybe_offset = parse_length(context, tokens.peek_token());
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if (maybe_offset.has_value()) {
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offset = maybe_offset.value();
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offset_provided = true;
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tokens.next_token();
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}
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}
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vertical = EdgeOffset { *to_edge(identifier), offset, true, offset_provided };
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} else if (identifier == ValueID::Center) {
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found_center = true;
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} else {
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return error();
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}
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continue;
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}
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tokens.reconsume_current_input_token();
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break;
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}
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if (found_center) {
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if (horizontal.has_value() && vertical.has_value())
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return error();
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if (!horizontal.has_value())
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horizontal = EdgeOffset { PositionEdge::Left, center_offset, true, false };
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if (!vertical.has_value())
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vertical = EdgeOffset { PositionEdge::Top, center_offset, true, false };
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}
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if (!horizontal.has_value() && !vertical.has_value())
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return error();
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// Unpack `<edge> <length>`:
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// The loop above reads this pattern as a single EdgeOffset, when actually, it should be treated
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// as `x y` if the edge is horizontal, and `y` (with the second token reconsumed) otherwise.
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if (!vertical.has_value() && horizontal->edge_provided && horizontal->offset_provided) {
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// Split into `x y`
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vertical = EdgeOffset { PositionEdge::Top, horizontal->offset, false, true };
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horizontal->offset = zero_offset;
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horizontal->offset_provided = false;
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} else if (!horizontal.has_value() && vertical->edge_provided && vertical->offset_provided) {
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// `y`, reconsume
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vertical->offset = zero_offset;
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vertical->offset_provided = false;
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tokens.reconsume_current_input_token();
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}
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// If only one value is specified, the second value is assumed to be center.
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if (!horizontal.has_value())
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horizontal = EdgeOffset { PositionEdge::Left, center_offset, false, false };
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if (!vertical.has_value())
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vertical = EdgeOffset { PositionEdge::Top, center_offset, false, false };
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return PositionStyleValue::create(
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horizontal->edge, horizontal->offset,
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vertical->edge, vertical->offset);
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}
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RefPtr<StyleValue> Parser::parse_background_position_value(ParsingContext const& context, Vector<StyleComponentValueRule> const& component_values)
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{
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auto tokens = TokenStream { component_values };
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// FIXME: Handle multiple sets of comma-separated values.
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return parse_single_background_position_value(context, tokens);
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}
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RefPtr<StyleValue> Parser::parse_background_repeat_value(ParsingContext const& context, Vector<StyleComponentValueRule> const& component_values)
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{
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auto is_directional_repeat = [](StyleValue const& value) -> bool {
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@ -3245,6 +3414,10 @@ Result<NonnullRefPtr<StyleValue>, Parser::ParsingResult> Parser::parse_css_value
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if (auto parsed_value = parse_background_image_value(m_context, component_values))
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return parsed_value.release_nonnull();
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return ParsingResult::SyntaxError;
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case PropertyID::BackgroundPosition:
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if (auto parsed_value = parse_background_position_value(m_context, component_values))
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return parsed_value.release_nonnull();
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return ParsingResult::SyntaxError;
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case PropertyID::BackgroundRepeat:
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if (auto parsed_value = parse_background_repeat_value(m_context, component_values))
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return parsed_value.release_nonnull();
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